sexual selection i - university of oxfordusers.ox.ac.uk/~grafen/lectpres/ss1.pdf · • modern...

Sexual Selection I

A broad overview

Charles Darwin with his sonWilliam Erasmus in 1842

Emma Darwinin 1840

A section ofDarwin’s “noteson marriage”,1838.

Lecture Outline

• Darwin and his addition to NaturalSelection– Difficulties with Natural Selection– Definition and idea– Humans

• Modern Study of Sexual Selection– Types of study– Focus on advantages to mate choice

Examples of problematic phenomenafor the theory of Natural Selection

• Peacocks’ tails• Blackbird song• Antlers in deer• Sexual dimorphism in many species• Mating display of Great Crested Grebe

Examples of problematic phenomenafor the theory of Natural Selection

• Peacocks’ tails (technically trains)• Blackbird song• Antlers in deer• Sexual dimorphism in many species• Mating display of Great Crested Grebe

Lecture Outline



Darwin (1871, p256):

“We are, however, hereconcerned only with that kindof selection, which I havecalled sexual selection. Thisdepends on the advantagewhich certain individuals haveover other individuals of thesame sex and species, inexclusive relation toreproduction.”

‘In relation to reproduction...’

• Being more noticed by, more attractive to,or more persuasive towards the oppositesex, and so gaining a mating advantage– Inter-sexual selection, ‘female’ choice

• Out-competing other members of the samesex in contests whose outcome determinesmating success– Intra-sexual selection, ‘male’ competition

Darwin’s criteria

• Age: juvenile vs adult• Sex: male vs female• Season: breeding season vs year-round• Use: specially displayed during mating?

Why does advantage in reproductionlead to exaggeration?

• ‘Disproportionate gains’– chooser makes an all or nothing choice

however small the difference is– all choosers may make the same choice

• Our similarity to targets of propaganda– any preference rule can be exploited– sensory systems have limitations

Natural Selection vs Sexual Selection


• Utilitarian, functional • Showy, elaborate


• Utilitarian, functional• Solves a problem

• Showy, elaborate• Impresses an audience


• Utilitarian, functional• Solves a problem• Sensible

• Showy, elaborate• Impresses an audience• Whimsical


• Utilitarian, functional• Solves a problem• Sensible• Economical

• Showy, elaborate• Impresses an audience• Whimsical• Wasteful


• Utilitarian, functional• Solves a problem• Sensible• Economical• Fixed

• Showy, elaborate• Impresses an audience• Whimsical• Wasteful• Changeable


• Utilitarian, functional• Solves a problem• Sensible• Economical• Fixed• Constructive

• Showy, elaborate• Impresses an audience• Whimsical• Wasteful• Changeable• Destructive


• Utilitarian, functional• Solves a problem• Sensible• Economical• Fixed• Constructive• Dull

• Showy, elaborate• Impresses an audience• Whimsical• Wasteful• Changeable• Destructive• Exciting

Darwin (1871, p256):

“We are, however, hereconcerned only with that kindof selection, which I havecalled sexual selection. Thisdepends on the advantagewhich certain individuals haveover other individuals of thesame sex and species, inexclusive relation toreproduction.”

Human traits possibly related to sexual selection

• Racial differences– skin colour– hair colour and texture

• Gender differences– physical– psychological

• Analogy to fashion in clothes andpersonal styles

Lecture Outline



Kinds of modern work (with one example)

• Field empirical– Møller on barn swallows

• Experimental– Gerhardt on tree frogs

• Comparative method– Eberhard on reproductive anatomy

• Concepts / Mathematical modelling– Lande on runaway processes

Modern topics

• Fireflies• Bird coloration• Anatomy of reproductive organs• Sperm competition• Pollen competition• Disease resistance

Modern topics

• Fireflies• Bird coloration• Anatomy of reproductive organs• Sperm competition• Pollen competition• Disease resistance• … and more, discussed by Marian

Dawkins later in the course

Eberhard (1985)

Population level effects...

• include polygamy, polyandry, leks andruts

• are discussed further later in the course

Lecture Outline



Modern study of sexual selection

• focusses mainly on preferences,• treats preferences as adaptive,• looks for a selective advantage to

preference,• and there are several possibilities.

Females might choose

• non-adaptively: because of sensory bias,species recognition.

• adaptively directly: for reasons of fertilisationability, fecundity, nutrition, parental ability,territory quality, avoiding venereal disease,social status of offspring

• adaptively through good genes: because ofinherited attractiveness of sons, or heritabledisease resistance, or more generallyheritable ‘quality’.

Some formal ideas

• Fisher’s runaway model• Lande’s model• Measuring Sexual Selection:

Lande/Wade/Arnold• Hamilton/Zuk• Handicaps & Signalling

Andersson (1994)

Literature

• C. Darwin (1871) The Descent of Man andSelection in Relation to Sex. Republishedin 1981 by Princeton University Press.

• Extracts in M.Ridley (1987) The EssentialDarwin. Unwin Hyman.

• M. Andersson (1994) Sexual Selection.Princeton University Press.

Lecture Outline



In summary,

• Darwin invented sexual selection toaccommodate certain kinds of factswithin his scheme, partly as a defenceagainst non-biological theories

• He established there was female choicebut said little about why

• We now assume selection is at work,and want to know why there is choice

sexual selection i - university of oxfordusers.ox.ac.uk/~grafen/lectpres/ss1.pdf · • modern...

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